Sample metering valve for a sample preparation system

ABSTRACT

A sample metering valve for both liquid samples and slurry samples with entrained gas bubbles. The sample metering valve for liquids is comprised of a piston with sample chambers formed in the side thereof said piston moving freely into and out of a cylinder having an aperture matching the size of the piston. Sealing is provided by a seal which has no dead volume which cold flows under a spring bias to maintain the seal under various operating conditions. The sample metering valve for slurry samples includes a syringe embodiment having separately moving piston and valve in a cylinder. The syringe valve sucks up sample with a piston, isolates the sample with the valve and compresses the entrained gas bubbles with the piston. Another embodiment uses three, three way valves which are coupled to a pump and a means to compress the sample. The valves are operated to suck a portion of sample up into the tubes connecting the valves, isolate the sample from the sample container, compress the sample using the compressed gas and trap a known volume of the compressed sample in the tube between valves. The trapped sample is then flushed out of the system with diluent to prepare the sample for analysis.

This application is a divisional application of U.S. Patent applicationSer. No. 942,201 (currently co-pending), filed on Dec. 16, 1986 bearingthe same title.

BACKGROUND OF THE INVENTION

The invention relates to the field of sample preparation for performingchemical assays. More particularly, the invention relates to the fieldof sample metering valves for extracting a known volume of a sample outof sample containing chamber or a process stream of materials to beassayed.

Typically in chemical processing facilities and chemical analysis labsit is desirable to perform chemical assays. Often this analysis is doneby liquid or gas chromatography, although many other methods of assayingsamples for their chemical composition also exist. Often, before anassay can be done it is necessary to prepare the sample by isolating aknown volume of the sample from a container of same or by isolating aknown volume of the sample from a process stream. The portion of thesample so isolated is then diluted by adding the isolated sample volumeto a known quantity of diluent to prepare a sample solution of a userdefined concentration for the assay.

The amount of sample so isolated must generally be a precisely knownvolume so that the concentration of the final sample solution can beprecisely controlled. There is a prior art valve which has been used toperform this function which is manufactured under the trademark ISOLOKby Bristol Engineering Company of Yorkville, Illinois. This valve uses aT shaped end on a piston within a cylinder. The cylinder is open endedand the piston's T shaped end forms a cap on the cylinder when thepiston is in the retracted position. When the piston is the extendedposition, the end cap of the piston is moved away from the end of thecylinder. The piston has a cylindrical shaped recess therein which isformed a distance up from the distal end of the piston. When the pistonis driven to the extended position, the recess is pushed out of thecylinder into the surrounding environment and fills with whatever mediumsurrounds the valve. The user of the ISOLOK valve takes a sample bycausing the piston to extend out into the surrounding environment suchthat the recess fills with the material which surrounds the valve.Typically, the piston is driven to the extended position by a pneumaticdrive arrangement or by a stepper motor. After the recess is filled, thedriving apparatus retracts the piston back into the confines of thecylinder. This causes the end cap of the piston to seal the end of thecylinder so that no material outside the walls of the cylinder can getinto the cylinder or the recess thereby isolating a known volume ofsample material in the recess.

The problem with the ISOLOK valve is in the sealing arrangement. Three Oring seals are used around the circumference of the piston both aboveand below the recess. These O rings are separated by small spaces, andengage the side walls of the cylinder in sealing engagement. The gapsbetween the O rings are themselves small recesses, and the gaps aroundthe portion of the piston below the recess are exposed to thesurrounding medium when the piston is in the extended position. Becauseof this fact, the gaps between the O rings around the portion of thepiston which is exposed fill with the medium which fills the main sampleisolation recess when the piston is put into the extended position. Whenthe piston is retracted into the cylinder, the material trapped betweenthe O rings effectively is part of the isolated sample and is of unknownvolume. If the isolated sample is then diluted by extending the pistonagain into a known volume of diluent, the isolated sample is releasedinto the diluent along with whatever sample is trapped between the Orings. The result that instead of a known concentration of sample indiluent, there is an unknown concentration of sample in the diluent.Further, the inaccuracy of the sample volume is not a constantdeviation. There are variations in the error which occur often enoughthat the predictability of the error is low. This can degrade theprecision of the assay.

The ISOLOK valve also is not well suited to dealing effectively withslurries or liquid samples with entrained gas bubbles. The gas bubblestake up volume which could otherwise be filled with liquid and therebydecrease the accuracy of prediction of the exact amount of liquid samplewhich has been isolated.

Thus a need has arisen for a valve which can accurately and repeatedlyisolate a known volume of a sample from a larger volume stored in acontainer or from a process stream and which can handle the situation ofentrained gas bubbles or foam in the sample chamber or process streamgracefully and with precision.

SUMMARY OF THE INVENTION

According to the teachings of the invention, there is disclosed herein asample metering valve which can repeatedly and accurately isolate aknown volume of sample from a larger volume of sample. The samplemetering valve of the invention includes an open end cylinder in whichthere is positioned a piston having a T shaped end. The piston slidesback and forth in the cylinder between an extended position and aretracted position. The T shaped end is sized so as to form a sealingplug in the open end of the cylinder. A cylindrical recess is formed inthe piston up from the sealing plug end and is placed on the piston suchthat the recess is exposed to the surrounding medium when the piston isin the extended position. This causes the recess to fill up with thematerial of the surrounded medium when the piston is extended. When thepiston is retracted, the material in the recess is isolated.

No O ring seals are used on the piston in the valve of the invention.Instead, there are two rings of relatively harder, less deformablematerial affixed to the walls of the cylinder in sealing engagement withthe side walls of the piston. These two rings are separated by acylinder of relatively softer, more deformable material such that thetwo rings of relatively harder material are in contact with the endsurfaces of the softer material. A spring is disposed inside thecylinder concentrically around the piston. This spring contacts the ringof relatively harder material at the end of the softer material farthestfrom the sealing plug on the piston. The purpose of the spring is toapply a bias force to the relatively harder ring to exert pressure onthe softer material to cause it to expand against the side wall of thepiston thereby forming a better seal. Because there are no gaps betweenthe relatively harder sealing rings and the relatively softer sealingcylinder, and because the intersections between the rings are notexposed to the surrounding medium when the piston is extended, no deadvolume is available to fill with unknown volumes of sample.

Typically, the piston is driven either by a pneumatic system or bystepper motors.

Another embodiment of a metering sample valve is a syringe type valve.This valve is especially useful in dealing with slurries with entrainedgas bubbles or foam. These bubbles of gas take up volume in an isolatedsample which can lead to inaccuracy in predicting the actual volume ofliquid which has been isolated in a metering valve. The syringe tubesample metering valve utilizes a cylinder with a piston therein and aseparately movable end plug. The end plug is moved to an open positionso that the surrounding medium may enter the cylinder. During filling ofthe valve, the piston is left in its retracted position to leave maximumvolume inside the cylinder available for filling by the sample. Afterthe cylinder sample volume is filled, the piston is separately moveddown toward the sealing end plug thereby compressing any gas bubblesentrained in or otherwise trapped in the sample volume of the cylinder.During this downward movement of the piston, the amount of movement ismonitored by a sensor. When the piston will move no more, the totalmovement is determined from the sensor or by reading the motor stepnumber in the case of a stepper motor drive for the piston. The totalvolume of liquid in the syringe valve is calculated by subtracting thevolume displaced by the movement of the piston from the total originalvolume of sample in the cylinder before the movement of the piston.

The sample may then be released by causing the end plug to unseal thecylinder and either letting the sample flow out or by pushing it out byfurther movement of the piston. With liquid samples, especially veryviscous samples, the syringe type embodiment has the added advantagethat the process of filling the cylinder sample volume with sample maybe speeded up by using the piston to draw up the sample into thecylinder by moving it away from the sealing plug from a positionadjacent to the sealing plug at the time the plug is opened.

The preferred embodiment of the sample metering valve for use in slurryand other sample situations where the volume consumed by gas bubblesexists is comprised of three, three way valves coupled to a samplepumping mechanism and a means to exert force on the sample to cause itto compress. A first three way valve (basically a Y valve) has itscommon port coupled to a fill pipe in a sample chamber, and a number 1port coupled to the number 1 port of another three way valve. Thisconnection forms a sample chamber between the valve mechanisms of thefirst and second valves. The number 2 ports of the two valves arecoupled together to form a bypass loop. The common port of the number 2valve is coupled to the common port of a third three way valve which hasone of its ports coupled to the sample pump and the other port coupledto the source of pressurized gas.

The valves are operated to couple the sampling pump to the fill tubeinto the chamber and the sample pump is driven to suck sample up throughthe first valve and out through the sample chamber until enough sampleis drawn to completely fill the sample chamber and excess sample isdrawn to account for the effects of compression. The first valve number1 port is then closed to isolate the sample in the sample chamber, andthe third valve is operated to couple the pressurized gas into thesample chamber to compress the gas bubbles in the sample to a smallvolume. Any means of compression will do. This includes operating thepump in reverse. The second valve is then operated to trap thecompressed sample between the first and second valving mechanisms, whichis a known volume. The sample pump is then used to empty the rest of thesample not so trapped from the lines and from the sample chamber and toclean out the lines and the sample chamber with solvent. The valves andpump are then operated to free the trapped sample and to pump a knownquantity of solvent through the lines and to push the trapped sampleinto the sample chamber where it may be pumped to any analysis device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of the sample metering valve of oneembodiment with the piston in the extended position.

FIG. 2 is a cross sectional view of the sample metering valve of theembodiment of FIG. 1 with the piston in the retracted position.

FIG. 3 is cross sectional view of the syringe type sample meteringvalve.

FIG. 4 is a cross sectional view of the state of the syringe type samplemetering valve as an aliquot of sample is drawn up into the valve.

FIG. 5 shows the state of the syringe type sample metering valve as thealiquot of sample is compressed to reduce the volume consumed by the gasbubbles.

FIG. 6 shows the state of the syringe type valve after the piston hasreached the point where the gas in the sample has been substantiallycompressed.

FIG. 7 shows the state of the syringe type valve after the valve hasbeen opened and the piston has been pushed down to the end of thecylinder to push out the sample liquid.

FIG. 8 is a diagram of the preferred embodiment of the sample meteringvalve.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, there is shown a cross sectional view of the samplemetering valve of the preferred embodiment. A piston 10 is disposedwithin an open ended cylinder 12. The piston is typically metal with achrome finish or is highly polished so as to have a smooth surface tominimize friction as the piston moves back and forth on the y axis. Thepiston has a T shaped end with a sealing plug 14 which has the sameoutside diameter D as the inside diameter of a seal 16 at the "open" end18 of the cylinder. The piston 10 is shown in the extended position. Inthe retracted position of the piston 10, shown in FIG. 2, the sealingplug 14 is pulled back into the opening in the cylinder 12 so as to bein sealing contact with the sealing ring 16.

The piston 10 has a sample collecting recess 20 formed therein a smalldistance along the y axis away from the tip 22 of the piston. Thepurpose of the sample collecting recess 20 is to capture a known volumeof material from the surrounding medium 24 when the piston is in theextended position. Therefore, the recess 20 must be machined orotherwise formed to be of a known volume and must be placed on thepiston 10 and sized so as to be at least partially exposed to thesurrounding medium 24. Preferably, the recess 20 will be placed andsized so as to be completely exposed to the surrounding medium 24 whenthe piston is in the extended position as shown in FIG. 1. The pistonmay be formed of other materials than metal such as teflon or otherplastic materials. This is true of the cylinder 12 also. The caveat onmaterial selection is that the materials selected for any component ofthe valve must be compatible with the intended environment in which thevalve is to be used so that the environment will not adversely affectthe materials and cause a valve failure. This is particularly true insampling of process streams.

A significant improvement over the prior art for the valve of FIG. 1resides in the sealing structure. This structure has no dead volume orrecesses which can inadvertently collect unknown volumes of sample whenthe piston is in the extended position. This is accomplished by theelimination of multiple O rings for sealing and substitution of aflexible, self compensating sealing arrangement using the property ofcold flow of malleable materials to adjust for differences in dimensionsof the various components with variations in temperature. The sealingstructure is comprised of two sealing rings 16 and 26 of relativelyharder materials with a smaller creep rate (non recoverable strain orpermanent pecentage deformation or cold flow) separated by and inabutting contact with a cylindrical seal 28 of malleable material of arelatively faster creep rate. A spring 30 applies a constant force tothe upper sealing ring 26 biasing it to move toward the sealing ring 16thereby putting the sealing cylinder 28 in compression stress. Thiscauses the cylindrical seal 28 to attempt to cold flow, i.e., expand inwhatever direction is available for expansion in response to thecompression stress. If there is any gap between the sidewalls of thepiston 10 and the cylindrical seal 28, the cold flow results in radialstrain in the cylindrical seal 28 which reduces or eliminates the gapthereby effecting a good seal. Changes in temperature which alter thediameters of the piston 10 and the cylinder 12 (possibly differentially)will not adversely affect the integrity of the seal. This followsbecause the cold flow strain adjusts for any temperature induced changesin gap size since the pressure exerted by the spring 30 is substantiallyconstant regardless of changes in temperature. Substantially less coldflow in the sealing rings 16 and 26 results because of their relativelyharder constitution.

No dead space results in the sealing structure of the invention sincethere are no gaps between the sealing rings 16 and 26 and thecylindrical seal 28. Further, the seals are affixed to the cylinder andnot to the piston, so the seals never are moved by the piston out intothe surrounding medium. No spurious, unknown quantities of sample can beaccumulated by the seals because of this structure.

The apparatus to move the piston may be any known force producingapparatus such as pneumatic or electrical devices. It is not necessaryin the preferred embodiment to know exactly how far the piston movessince the the sample volume is fixed in the recess 20. It is onlynecessary to know that the piston has been moved to its extendedposition or to its retracted position.

In the preferred embodiment, the sealing rings 16 and 26 are TEFLONpolymer impregnated with glass, graphite or some other material whichmakes the TEFLON polymer than pure TEFLON polymer. The sealing cylinder28 is pure teflon, and has a higher degree of deformability than thesealing rings 16 and 26. These material selections are not critical tothe invention however, and any material which is chemically inert, has alow coefficient of friction and which can cold flow will be acceptablefor the sealing cylinder 28. The same is true for the material selectionof the sealing rings 16 and 26 except that the material must berelatively less deformable than the sealing cylinder 28, or must becapable of being made so with suitable alloying or other techniques.

SYRINGE VALVE

Referring to FIG. 3 there is shown a cross section of a syringe typesampling valve which is advantageous for use in sampling slurry typesamples and samples with entrained gas bubbles or surface foam bubbleswhich take up space. The syringe type valve is comprised of twoseparately movable sections. A valve 30 driven by a shaft 32 opens andcloses a port 34 in a cylindrical valve body 36. The valve is driveninto and out of sealing engagement with a valve seat formed on the port34 by a valve drive system 38 of known construction. The details of thevalve drive system 38 are not critical to the invention, and anymechanism which is capable of causing the valve 30 to move into and outof sealing engagement with the valve body 36 will suffice for purposesof practicing the invention.

The other portion of the sample valve which moves independently is thepiston 40. The piston 40 moves back and forth inside the body 36 of thevalve along the y axis under the influence of a piston drive system 42.The piston 40 has a hole therein through which the shaft 32 of the valve30 passes. The clearance between the shaft 32 and the walls of the holein the piston 40 should be such that substantial sealing engagementbetween the shaft and the walls of the hole in the piston is maintained.This necessary because the piston 40 will be forced downward in thenegative y direction to compress any gas in the sample to minimalvolume, and pressure will result in the chamber which could escapearound the valve shaft 32 unless a sealing fit is maintained. Because ofthis sealing engagement, the materials selected for the valve shaft 32and the piston 40 should not only be able to withstand the forcesinvolved, but should also have low coefficients of friction.

The details of the piston drive mechanism are not critical to theinvention except that the piston drive mechanism 42 should be such thatthe amount of movement of the piston 40 can be accurately determinedduring the compression stage of operation of the sampling valve. Astepper motor system or a motor system or pneumatic system coupled withan optical or other type of sensor which can accurately determine theamount of movement of the piston 40 will suffice for purpose ofpracticing the invention. Any type of motive power source which can movethe piston 40 will suffice, and any type of sensor or other device whichcan detect and signal the amount of movement of the piston 40 willsuffice for purpose of practicing the invention.

Because the piston drive system 42 and the valve drive system must worktogether in a coordinated fashion to process samples with gas bubbleswhich must be compressed, a communication link or control bus 44 isshown to carry the proper signals from one unit to the other to allowthe needed cooperation to occur.

Referring to FIGS. 4 through 7 there are shown a sequence of states ofthe two moving parts of the syringe to take a sample, compress the gasvolume to a small volume and measure the sample volume after compressionand to release the sample.

FIG. 4 shows the state of the syringe valve as it is filled with sample.In this state, the valve operation mechanism 38 forces the valve 30 openby pushing the valve shaft 32 downward in the negative y direction.Simultaneously or after the valve 30 is unseated, and the aperture 34 isopened, the piston 40 is pulled upward in the positive y direction bythe piston drive mechanism 42. This increases the volume of the"chamber" defined by the end 46 of the piston, the inside walls of thecylinder 36 and the end of the cylinder 36 having the aperture 34. Theresultant lowering of pressure causes the sample slurry 48 to be suckedup into the chamber of the syringe valve.

Referring to FIG. 5 there is shown the state of the syringe valve as theentrapped gas bubbles are compressed. The piston 40 is shown at itsfully retracted position. The valve control mechanism 38 has closed thevalve 30 to block any flow of sample into or out of the sample chamberby pulling the valve shaft 32 upward in the positive y direction. Thepiston drive mechanism 42 then pushes the piston 40 downward in thenegative y direction to begin compressing the gas bubbles in the sample.The piston drive mechanism 42 must be such that the position of thepiston 40 in its retracted position P₁ can be noted so that the totalmovement of the piston downward in the negative y direction may becalculated. The total travel of the piston 40 during the compressionmust be capable of accurate measurement by the piston drive mechanism42. Typically, a stepper motor system would be used for the piston drivemechanism 42 with a computer acting through a motor controller interfacechip controlling the motor. To implement the movement of the pistonalong the positive y direction, the computer sends a motor movementcommand to the motor control interface chip (not shown) in the pistondrive control mechanism 42. Typically, this command would include thenumber of steps to move or the step address for the position P₁ with acommand to start moving in the positive y direction and to stop movingwhen the designated step address is reached for the position P₁.Alternatively, the computer can send a command to start moving in thepositive y direction and then continually query the motor controlinterface chip to read the current step number as the movementprogresses. As the data is received from the motor controller interfacechip, the current step number is compared with the desired step number,and a stop command is issued to the motor controller chip when thepiston reaches step P₁. All such embodiments would be equivalent.Alternatively, simpler pneumatic equipment may be used to move thepiston 40 between upper and lower stops with the upper stop being theposition P₁, and the lower stop being the bottom of the cylinder 36. Tostart the piston moving downward as in FIG. 5, the computer would issuea start command and direction data to the motor controller interfacechip to cause the piston to move in the negative y direction. Thismovement would continue until all the gas bubbles were compressed. Adetector mechanism to detect the amount of "backpressure" on the pistonmay be used to determine when the amount of back pressure is sufficientto indicate that all the gas bubbles have been compressed sufficiently.What is a sufficient amount of compression must be determined by theuser, and will depend upon the level of accuracy in the final dilutedsample concentration desired. For very accurate sample concentrations,more compression is used to insure the final volume of the chamber issubstantially all sample liquid.

Referring to FIG. 6, there is shown the state of the syringe samplevalve after the compression is done. The piston 40 is shown in its finalposition after sufficient compression of the gas bubbles in the sampleliquid has occurred. The valve 30 is left closed during the compressionstroke by the piston 40. The operation of the piston drive mechanism 40to achieve the state shown in FIG. 6 depends upon the type of pistondrive mechanism 42 which is used. As mentioned above, the particulardesign for the piston drive mechanism 42 is not critical to theinvention as long as the criteria stated above can be met. Those skilledin the art will appreciate many different mechanisms which could be usedfor the piston drive mechanism 42. The same is true for the valvecontrol mechanism 38. The design for computer controlled stepper motorsystems and the programming for same is well understood by those skilledin the art. For a computer controlled stepper motor system, the movementof the compression stroke could be accomplished as follows. When asufficient amount of back pressure is detected or adequate compressionis otherwise detected, the computer issues a stop command to the motorcontroller interface chip, and the piston stops at the position P₂. Thecomputer then reads the step number for the current piston position.This step number is compared to the step number for the position P₁, andthe difference is then converted into the distance of travel for thepiston along the y axis from position P₁ to position P₂. This distancemay then be converted to the final volume of the sample shown in FIG. 6.This is done by calculating the volume difference between the volume atposition P₁ and the volume at position P₂ using the amount of linearpiston travel between these positions.

In other embodiments, the valve controller 38 is programmed or otherwiseconstructed to apply a certain, user defined force to the valve 30 tokeep it closed. The piston controller 42 is then programmed to applydownward pressure to the liquid in the sample chamber until the pressurein the liquid is enough to force the valve open. This valve openingevent is detected by sensing movement of the valve shaft 32 or bydetecting fluid escaping from the sample chamber optically or otherwise.The amount of compression can be controlled in this embodiment bycontrolling the amount of force applied to keep the valve 30 closed. Thetime of the valve opening is signaled to the controller by an interruptfrom whatever sensor (not shown) is used to detect the opening of thevalve 30. This allows the controller to either stop moving the piston 40or to continue using the piston until the volume of liquid in the samplechamber is the user defined amount. This final volume may be determinedby continual query to the stepper motor controller to determine thecurrent step position and comparison to the step number calculated forthe volume desired by the user.

In still other alternative embodiments, the user may program the piston40 to move to a position P₁ which represents a predetermined volumedifferential over the volume represented by position P₂. The volumerepresented by position P₁ will be greater than the volume representedby position P₂ by an amount determined by the user from thecharacteristics of the slurry or other sample being used. That is, theuser will, for a given type of sample, know the approximate volume ofthe gas in the sample which must be compressed. This volume of gas willbe assumed to be true for all samples of that particular type of sampleliquid, and will represent the volume differential between positions P₁and P₂.

Those skilled in the art will appreciate other embodiments for or waysof operating the piston controller 42 and the valve controller 38 toaccomplish the gas compression in moving the piston from the position P₁to the position P₂. As long as these other embodiments are capable ofcompressing the gas so that the final volume of liquid in the samplechamber shown in FIG. 6 is substantially all liquid, then theseembodiments are regarded as equivalents.

FIG. 7 shows the final stage of the slurry slampling process after thesample liquid in the sample chamber of FIG. 6 has been pushed out of thevalve. To accomplish this, the valve controller 38 pushes the valve 30open in any know fashion, and the piston controller 42 pushes the piston40 down the negative y axis until the piston 42 reaches the valve seatend of the cylinder 36.

FIG. 8 shows the preferred embodiment for the sample metering valve forslurry or other samples where the volume consumed by gas bubbles is tobe eliminated or minimized. The sample metering valve is actuallycomprised of three, three way valves labelled A, B and C in FIG. 8. Eachthree way valve is a Y connection with a valve gate such as the gates 50and 52 in valve A, and each valve A through C has three ports labelled 1through 3. The gate valves in each valve operate so that at anyparticular time only one of ports 1 or 2 is coupled to port 3. Theconnections are as shown in FIG. 8 for the sample metering valve of thepreferred embodiment.

The operation of the system to take a sample is as follows. A sample cup54 is filled with sample 56. Ports 1 on valves A and B are thenactivated (opened). and port 2 of valve C is activated. A sample pumpcoupled to port 2 of valve C is then turned on to pump liquid in thedirection of arrow 60. This draws sample up into the fill tube 62 andthrough ports 3 and 1 of valve A, pipe 64, ports 1 and 3 of valve B,pipe 66, ports 3 and 2 of valve C, pipe 68, pump 70 and empty pipe 72.Any pumping mechanism or system will suffice for practicing theinvention as long as the accuracy of delivery at least in the directionof flow into the sample cup 54 can be controlled. An accurate pump isneeded for dilution, but is not needed for sample isolation.

The pump 70 must be pumped long enough to completely fill the pipe 64and at least partially fill pipe 66 with enough sample such that whenthe sample is compressed, the pipe 64 remains filled to capacity. Thesample chamber of known volume in the embodiment of FIG. 8 is the pipe64 plus whatever volume exists in the valves A and B up to the valveplates.

After filling the sample chamber, valve A, port 2 is activated to trapthe sample in the pipe 64, and valve C, port 1 is activated to couplepressurized gas into pipe 66. This pressurizes the liquid and gas in thepipes 66 and 64 and thereby compresses any gas bubbles in the pipes 64and 66 down to zero or small volume. The volume of material in thesample chamber is substantially all liquid by virtue of thispressurization of the lines. Next, valve B, port 2 is activated therebyisolating the sample in the sample chamber 64 between valves A and B.The pump 70 is then activated to pump the remaining sample 56 in thesample cup and any remaining untrapped sample in pipe 66 out of thesystem through pipe 72. That is, sample is pumped up through fill pipe62, ports 3 and 2 of valve A, pipe 74, ports 2 and 3 of sample valve B,pipe 66, ports 3 and 2 of valve C, pipe 68, pump 70 and out pipe 72.

Pipe 72 in the preferred embodiment may be coupled alternately to asource of solvent and to a waste dump. The pump 70 is then activated topump solvent in the direction of the arrow 76 to flush out the pipes 66,74 and 62 and to wash out the remaining sample from the sample cup 54.The pump 70 is then reversed to pump out the solvent in the system andthe sample cup in preparation for the dilution.

Next, ports 1 of valves A and B are activated, and the pump 70 isactivated to pump in the desired amount of diluent to get the desiredsample to diluent concentration. The diluent pumped in the direction ofthe arrow 76 flushes the trapped sample out of the pipe 64 down into thesample cup 54. Since the volume of trapped sample is relativelyprecisely known, good accuracy of the sample concentration may beobtained. Serial dilutions are also possible by repeating the abovesteps several times to get successively weaker concentrations.

Control logic 80 supplies control signals to all valves and the pump 70via control bus 78. The control logic 78 may be a programmed digitalcomputer, dedicated combinatorial logic or any other circuit which cancause the above identified algorithm to work. The details of such logicwill be apparent to those skilled in the art given the above descriptionof how the system is supposed to operate, and no furhter details will begiven here.

Although the invention has been described in terms of the preferredembodiment and alternative embodiments disclosed herein, those skilledin the art will appreciate other embodiments which accomplish the sameresult and which do not depart from the spirit of the invention. Allsuch alternative embodiments are intended to be included within thescope of the claims appended hereto.

What is claimed is:
 1. A sample metering valve apparatus for sampleshaving gas bubbles therein comprising:a cylinder having at least oneaperture in one end; valve means for moving into and out of sealingengagement with said aperture under the influence of forces applied to avalve shaft; piston means in said cylinder for movement thereinindependently of movement of said valve means and having an aperturetherein through which said valve shaft passes.
 2. The apparatus of claim1 further comprising valve drive means for applying force to said valveshaft at a user definable level.
 3. The apparatus of claim 2 furthercomprising piston drive means for applying force to said piston means tocause it to move within said cylinder.
 4. The apparatus of claim 3wherein said piston drive means and said valve drive means include meansfor causing the valve means to be opened, the piston means to drawliquid into said cylinder, the valve means to close and the piston meansto move in the direction to compress any gas in said liquid.
 5. Theapparatus of claim 4 wherein said valve drive means includes means tocause said valve means to be held closed with a user definable force andmeans in said piston drive means to cause said piston means to applysufficient compressive force until said valve means opens and to measurethe amount of remaining volume in said cylinder after said compression.6. The apparatus of claim 3 wherein said piston drive means and saidvalve drive means include means for causing the valve means to beopened; for causing the piston means to move a predetermined, userdefinable amount in a direction to draw liquid into said cylinder; forcausing the valve means to close; and for causing the piston means tomove a predetermined, user definable amount in a direction to compressany gas in said liquid.